To ເມືອງໂພນທອງ Muang Phon Thong 'including' ປາກອືມ Pak Oum & ນາຊອນ Na Son (70,000kip) - 1000, daily. This new service was announced in a notice at Luang Prabang northern bus station (08 December 2009). There was also a sign indicating the berth for 'ວັງຊຽງ Vang Xieng & ປາກອືມ Pak Oum'. No idea where this particular Phon Thong & Pak Oum are, but there is a Vang Xieng town along a minor road that branches off from HWY 1C at Phou Thid Pheung (between Nong Khiaw & Vieng Kham) & leads to a Na Son, a locals-only Laos-Vietnam border crossing (top part of this map). Anyone with more info on this route, please update the cat.

(info from 17 October 2008)
Ask at any shop/restaurant/guesthouse at the 3-way junction, locals will point out the man who will sell you tickets & flag down the correct bus for you.
To Vientiane - any bus passing through, 80,000kip
First Xieng Khouang-Vientiane bus passes through at ~1030.
First Luang Prabang-Vientiane bus passes through at ~1130.